Using video intensification fluorescence microscopy and rhodamine-labeled triiodothyronine, the uptake of T3 by cultured mouse fibroblasts was investigated. A saturable sequestering of labeled-hormone on the cell surface was observed leading to the formation of endocytic vesicles. By comparison with the entry of alpha 2-macroglobulin into the same cells it was concluded that thyroid hormone was accumulated and endocytosed through coated pits.